Internal combustion engines have a plurality of cylinders which are each coupled to a camshaft and to a crankshaft. The crankshaft takes up piston forces of the individual pistons of the cylinders, said forces being conducted via respective connecting rods, and converts the piston forces into a torque. In this context, the camshaft is held in a defined position using an actuator element.
In modern internal combustion engines, the camshaft position is repeatedly re-adjusted, even during operation, using an actuator element. This requires an actuator element which knows the precise position of the camshaft or determines the ideal camshaft setting on the basis of characteristic diagrams. The required setpoint setting of the camshaft position can then be held with an actuator element. When modern actuator elements are controlled, the maximum achievable control speed is limited owing to the lag times which occur and the delayed response.
Because of the lag times and the delayed response, the controller which controls the actual setting of the camshaft cannot be supplied with an integral component, since otherwise an unstable system would be produced. Therefore, in modern camshaft-adjustment devices a certain maximum control error of the actuator element is permitted, below which error the controller does not react.
If the actual setting of the actuator element, and therefore of the camshaft, cannot be measured continuously but instead is determined by means of sampling, problems may occur, since even after repeated engagement of the controller the setpoint setting is not reached and instead a quasi-steady-state is set.
This resulting drift, which does not originate from changed operating parameters of the actuator element, is currently compensated using a load-dependent and rotational-speed-dependent characteristic diagram.
EP 1 272 741 B1 discloses a method for finding the holding pulse duty factor which is required to hold an actuator element in a desired setpoint setting. This method essentially relies on the fact that the minimum of the through-flow characteristic curve is determined by the operating parameters of the solenoid valve. In this context, an actuator element can be moved between two end settings. The actuator element is acted on in one end setting, and can be moved to another end setting by activating an adjustment unit. In this context, the actual setting of the actuator element is determined by sampling. The actuator element is then actuated by means of a pulse-width-modulated signal, and is held in a setpoint setting using an actuation with the holding pulse duty factor. If a control error is continuously exceeded despite a repeated control intervention, the holding pulse duty factor is adapted.
In modern internal combustion engines with a camshaft-adjustment system, new dependencies arise owing to current tendencies. These new dependencies emerge, in particular, for example as a result of a drop in oil pressure, more compact design of the camshaft adjusters, the drive of additional components via the camshaft and variable camshaft geometries.